This invention relates generally to packaging machines for wrapping stretch film around products supported upon trays, and more particularly, to a package wrapping method and machine wherein a single fixed length of wrapping material is initially drawn, with the ultimate film length used to wrap each package being determined by the height of the package and the length of the package, the latter of which is used to select the time during the wrapping operation at which the film is severed from a continuous source of film.
A variety of machines are available for wrapping film about trayed products, for example meats and produce in supermarkets, to prepare attractive packages for consumer display. Two early examples of such machines are illustrated in U.S. Pat. Nos. 3,662,513 and 3,967,433, both of which disclose using only a fixed length of stretchable film. Such fixed film length wrapping wastes film and can potentially create unsightly clumps of film on the bottom of the smaller sized packages resulting in weak bottom seals and leaking packages.
To overcome the problems created by using a single fixed length stretchable film sheet for wrapping all packages, film wrapping machines have been developed which permit the selection of a variety of film lengths to accommodate varying package sizes. An example of an automatic film length selection mechanism for a film wrapping machine is disclosed in U.S. Pat. No. 4,510,731. In this patent, an adjustable mechanism is controlled in response to the measured length and height of a package to control the film drawing stroke of a film gripper to select the appropriate length of film for wrapping the package. Unfortunately, the adjustable mechanism is complicated mechanically and requires an adjusting control motor, sensing switches and control electronics to vary automatically the lengths of film drawn into the machine. Accordingly, while the adjustable mechanism is a substantial improvement over the fixed film length wrapping of the prior art, it does entail expense both for the initial components and also for continuing maintenance, particularly as the mechanism ages.
Another film sheet sizing arrangement is disclosed in French Publication No. 2,410,601 wherein a film sheet is held across the path of an obliquely raised elevator. In the French publication, a package is placed on the elevator and obliquely raised into and through a plane defined by the film to thereby draw any additional film which is required by the height of the package from a continuous source of the film. The obliquely raised elevator comprises a collapsible surface which is sequentially replaced by a film underfolder to thereby support the package and permit the elevator to be lowered for the next package to be wrapped. The film sheet is sized by severing the sheet only after the under folder has been entirely inserted under the package to position the severed film end at a film holding mechanism for the next package. The leading and trailing edges of the film are brought together to form an overlapping sleeve arrangement for wrapping the package. Unfortunately, the film sheeting arrangement disclosed in the French publication results in a double overlap sleeve wrapping which wastes film relative to the wrapping arrangement of the present wrapping machine, and still entails complicated mechanical arrangements which are expensive from both a production and maintenance standpoint.
Accordingly, it is apparent that the need exists for an improved package wrapping method and machine which eliminate the complicated prior art sheeting arrangements, yet still provide film lengths selected in response to package dimensions to conserve film and provide attractive film wrapping characteristics.